This invention relates to solar energy converting apparatus and, more particularly, to an improvement in such apparatus wherein a solar radiation collector is formed of a plurality of parabolic-shaped parallel troughs which are pivotable about longitudinal axes.
The energy contained in solar radiation provides a useful source which can be exploited so satisfy much of the requirements which now are satisfied by other energy sources, such as fossil fuels, nuclear energy, hydroelectric power sources, and the like. Ideally, received solar radiation can be converted into thermal energy and then used for numerous other purposes. For example, the heat derived from solar radiation can be used, theoretically, to produce steam which, in turn, can serve as a source of motive power. As another example, such generated steam can be used to drive conventional electrical generating equipment. As a still further example, the heat derived from solar radiation can be suitably stored and used to heat dwellings and other edifices, as well as a source of hot water.
Many prior art solar energy converting devices are known to derive heat from solar radiation. Typically, such apparatus includes a heat collecting plate which is darkened, or otherwise suitably treated, and which receives impinging solar radiation. As is recognized, the collecting plate is heated by the received radiation, and the heat produced thereby is transferred to suitable heat transfer fluid. Generally, the heat transfer fluid, which may be a liquid, is adapted to flow in suitable conduits, or channels, disposed beneath the heat collecting plate. Typically, such fluid conduits are formed of conventional pipes adjacent the collecting plate to thereby permit heat to be transferred directly from the plate to the fluid. The heated fluid then is permitted to flow to a suitable storage vessel, or the like, or to other devices wherein the heat is utilized. In many applications, the heat transfer fluid flows through a closed loop so as to receive heat from the collector plate and to subsequently discharge the heat to achieve the particular purpose for which the system is designed. In other systems, the heat transfer fluid is gaseous but, nevertheless, is provided for the same purpose as the aforedescribed heat transfer liquid. Thus, the heated gas may be used in, for example, a hot air heating system, or may be used to drive turbine or other devices.
Most prior art systems which use a flat or substantially flat collecting plate must position that plate with respect to the sun so as to receive optimum radiation. Of course, maximum solar radiation is received if the collecting plate is caused to track the trajectory of the sun, thus requiring diurnal tracking. Such tracking requires complex and costly machinery and significantly adds to the expense of solar energy conversion. Moreover, such prior art systems have not been capable of generating a significant amount of heat to thus justify the use of such complex and costly apparatus. Indeed, in those geographic areas wherein less than a maximum amount of sunshine is available on a daily basis, prior art solar energy converting apparatus has not found favorable application.
A marked improvement over conventional solar energy converters has been proposed by Roland Winston. Winston's proposal is to add a solar energy concentrator over the heat collecting plate so as to maximize the concentration obtainable by a solar energy converter. In particular, the radiation concentrator consists of a parabolic-shaped light-collecting trough. The axis of the parabola which forms the trough is inclined at an angle .theta..sub.max to the optic axis thereof. The angle .theta..sub.max is known as the half angle of maximum beam divergence. It has been found that, with a parabolic-shaped collecting trough, if the entrance aperture to the trough has a diameter d.sub.1 and the exit aperture from the trough has a diameter d.sub.2, then the maximum permissible concentration factor is equal to d.sub.1 /d.sub.2 +(n.sub.2 /n.sub.1)(1/sin.theta..sub.max), where n.sub.1 and n.sub.2 are the indices of refraction at the entrance and exit, respectively. If such indices of refraction are equal, then the height L of the trough is defined as L=(1/2)(d.sub.1 +d.sub.2)cot.theta..sub.max. A complete mathematical discussion of this parabolic-shaped light collecting trough is set forth in "Principles of Solar Concentrators of a Novel Design" by Roland Winston, 16 Solar Energy 89-95 (Pergamon Press 1974).
In a radiation concentrator of the type proposed by Winston, solar radiation can be concentrated by a factor of 10, without diurnal tracking. That is, a sufficient amount of heat can be produced by a stationary solar energy converter. If such an energy converter is provided with plural parallel troughs, the positioning thereof with respect to the sun need be modified only to account for seasonal changes. However, even with this improved radiation concentrator, it is necessary to mount the concentrator on a suitable support whereby the optic axis of each trough exhibits a suitable angle with respect to the vertical. This angle generally is equal to 45.degree., but may encompass a range of from 30.degree. to 60.degree.. Such angulation requirements of the improved solar concentrator thus limits its ready incorporation into existing structures or into planned edifices. Consequently, the concentrator generally cannot be incorporated into, for example, a building. Rather, it must be annexed to a structure with the attending increase in cost and non-integral design. That is, the materials which are used in such a solar energy converter cannot be used as part of the building structure, such as the roof, and thus increase the total cost of the building.
As another disadvantage of this type of improved solar energy converter, a substantial amount of heat loss will occur during those hours when no solar radiation is received, e.g., during the night. To minimize such heat loss, additional thermal insulating devices must be used to cover, or shield, the radiation concentrator. Also, even though only minimal angular adjustment need be effected, as during seasonal changes, nevertheless, it is necessary to provide the requisite adjusting machinery. This has the same disadvantages as found in the conventional solar energy converters of the type described hereinabove.